This disclosure relates to active handoffs between radio networks.
Cellular wireless communications systems are designed to serve many access terminals distributed in a large geographic area by dividing the area into cells, as shown in FIG. 1. At or near the center of each cell 102, 104, 106, a radio network access point 108, 110, 112, also referred to as a radio node (RN) or base transceiver station (BTS), is located to serve access terminals 114, 116 (e.g., cellular telephones, laptops, PDAs) located in the cell. Access terminals (AT) are sometimes referred to as mobile stations (MS) or user equipment (UE). Each cell is often further divided into sectors 102a-c, 104a-c, 106a-c by using multiple sectorized antennas. An RN is identified by one or more of several properties, which may include the offset of a pseudonoise pattern in its pilot signal (PN offset), a frequency, an IP address, or a SectorID. In each cell, that cell's radio network access point may serve one or more sectors and may communicate with multiple access terminals in its cell.
When an access terminal moves from one sector or cell to another and control of the access terminal is transitioned between different network elements, the transfer is referred to as handoff. If the access terminal has a call in progress during handoff, the handoff is said to be active. Co-pending patent application Ser. No. 11/037,896, filed Jan. 18, 2005, and titled Radio Network Control, also assigned to Airvana, Inc., described active handoffs in partially-connected radio networks. The type of handoff described in that application is now standardized as the A16 interface in the TIA-878-B standard. According to the standard, A16 handoff is a hard handoff, such that during the handoff, the source radio network controller cannot add the target radio node to its active set, and the target radio network controller cannot add the source radio node to its active set.